The present invention relates to a process for manufacturing a cold-rolled steel strip for deep drawing.
At the present time, steel strips intended for drawing operations are generally cold-rolled steel strips, which have very favourable properties in this respect. However, the manufacture of these cold-rolled strips involves various thickness-reducing and heat-treatment operations which increase their cost.
The use of hot-rolled steel strips for drawing operations, in replacement for conventional cold-rolled strips, is consequently arousing increasing interest, both in terms of manufacture and for users.
It is well known that steels intended for deep drawing are mild steels, that is to say steels whose carbon content is between 0.02% and 0.08% by weight and whose manganese content is between 0.1% and 0.4% by weight.
According to the usual practice, mild steels are hot-rolled in the austenitic region and the temperature at the end of rolling is higher than the conversion temperature Ar3, that is to say a temperature which is generally between 820xc2x0 C. and 880xc2x0 C. However, the possibilities for using these conventional hot-rolled strips are very limited, on account of their random texture and their poor drawability. In addition, it is impossible in practice to manufacture hot-rolled thin strips by this conventional method. The reason for this is that the thinness of the strips causes them to cool rapidly, even during hot rolling, such that it is not possible to carry out the finishing rolling in the austenitic region in order to obtain a microstructure that is favourable for the subsequent shaping operations by deep drawing.
Conventional Method Currently Used
At the present time, steels for deep drawing of the type FePO1 and FePO3, the designations relating to the European standard EN 10130, use steels with a low carbon content (0.02 less than C. less than 0.08% by weight) and a low manganese content (0.1 less than Mn less than 0.4% by weight), which undergo hot is rolling in the austenitic region and are wound at high temperature (680xc2x0 C. less than T less than 750xc2x0 C.). These steel strips are then cold-rolled with a reduction ratio of between 65% and 80% and undergo continuous annealing.
Table 1 indicates the minimum mechanical properties required in the context of the two types of commercial steel for deep drawing FePO1 and FePO3.
FePO1 and FePO3 are the types of steel as defined in European standard EN 10130 relating to the qualities of commercial steels for deep drawing;
YS (MPa) is the yield strength expressed in megapascals; TS (MPa) is the tensile strength expressed in megapascals;
Eltot (%) is the total elongation at break expressed in
R90 is the Lankford parameter measured at 90xc2x0 relative to the direction of rolling.
During the abovementioned hot rolling, the winding of the steel strip at a high temperature, that is to say a temperature of between 680xc2x0 C. and 750xc2x0 C., is carried out in order to obtain in the hot strip the total precipitation of the N in the form of coarse nitrides, this condition promoting the control of the texture of the strip during the recrystallization annealing.
After cold rolling, the strip undergoes a continuous annealing comprising heating at a rate of about 10xc2x0 C./s up to an annealing temperature which is in the ferritic region, that is to say less than or equal to 720xc2x0 C., and maintenance at this temperature for about 1 minute, followed by cooling at a rate of between 10 and 20 C./s down to the overageing temperature. This overageing treatment is necessary in order to obtain a strip whose microstructure comprises a sufficiently small amount of soluble carbon so as to have a small ageing index (AI). In general, the time for which the strip is held at an overageing temperature of between 350xc2x0 C. and 500xc2x0 C., which is essential in order to obtain an adequate precipitation of carbides, is several minutes.
Drawbacks of the Abovementioned Conventional Method
A relatively low annealing temperature ( less than 720xc2x0 C.) which gives rise to a fairly fine-grain microstructure and consequently promotes the presence of nucleation sites for Fe3C. during the slow primary cooling, that is to say from the annealing temperature down to the overageing temperature. The conventional values for the cooling rates in the case of steel strips 0.8 mm thick subjected to a conventional cooling with jets of gas are between 5 and 15xc2x0 C./s. Consequently, an appreciable amount of C is already precipitated at the start of the overageing treatment, and this results in a harmful smaller supersaturation effect and thus slower carbide precipitation kinetics at the overageing temperature.
An obligation for relatively long maintenance at the overageing temperature, of about from 3 to 5 minutes, which is the consequence of the above comment, but is necessary in order to reduce the amount of interstitial carbon present in the final product to below a value which is low enough to avoid any subsequent ageing.
After a conventional overageing treatment, that is to say with maintenance at 400xc2x0 C. for 3 to 5 minutes, in a continuous annealing line using the conventional technique of cooling with jets of gas, the ageing index of the product obtained is about 50-60 MPa. Given that a product is defined as being ageing-free when its ageing index is less than 30 MPa (see the following: K. Ushioda et al., Metallurgical Investigation for producing non-ageing deep-drawable LC. AK-steel sheets by continuous annealing, Developments in the annealing of sheet steels, edited by R. Pradhan and I. Gupta, 1992, pp. 261-285), which corresponds to an absence of a flow threshold (Luders strain) in a simulation of ageing in the form of a maintenance at 100xc2x0 C. for 1 hour. The abovementioned operation of maintenance at 100xc2x0 C. for 1 hour is representative of a storage operation for 3 months at 30xc2x0 C. usually carried out in reality by the sheet manufacturer before dispatch to the user. This means that the products thus obtained, and thus also the commercial types FePO1 and FePO3 for deep drawing thus obtained, are sensitive to strain ageing.
To avoid the abovementioned drawbacks, the present invention proposes a process for manufacturing a hot-rolled steel strip for deep drawing of the FePO1 and FePO3 type.
In accordance with the invention, a process for manufacturing a cold-rolled steel strip for deep drawing, of between 0.3 mm and 1 mm thick, for application to steels with a low carbon content (0.02 less than C. less than 0.08% by weight), a low manganese content (0.1 less than Mn less than 0.4% by weight), S less than 0.015% by weight, Si less than 0.1% by weight, P less than 0.08% by weight, Al less than 0.05% by weight, Nb less than 0.02% by weight and Ti less than 0.03% by weight, in which a steel slab of the abovementioned type is subjected to hot rolling in the austenitic region with winding at the end of the hot rolling at high temperature (680xc2x0 C. less than T less than 750xc2x0 C.), the said hot-rolled strip being subsequently subjected to cold rolling with a reduction ratio of between 65% and 80%, and finally undergoing a heat treatment of annealing and overageing, is essentially characterized in that the steel strip is heated at a heating rate Vh of between 150xc2x0 C./s and 1000xc2x0 C./s up to the annealing temperature Ta of between 650xc2x0 C. and 750xc2x0 C., the said strip is maintained at the annealing temperature for a time ta of between 1 and 20 seconds, and the said strip is cooled at a cooling rate Vc of between 100xc2x0 C./s and 500xc2x0 C./s down to an overageing temperature Toa of between 150xc2x0 C. and 450xc2x0 C.
According to one embodiment of the process, which is the subject of the present invention, the steel strip is heated until it reaches the annealing temperature Ta by induction, preferably by creating a longitudinal induced flux.
Heating the steel strip in this way presents the advantage of great flexibility in the choice of the temperature Ta, and also in the feasibility as regards very high heating rates Vh. In addition, this type of induction heating improves the production efficiency of the process and broadens its field of practical application.
According to another embodiment of the process which is the subject of the present invention, the cooling of the strip from the annealing temperature Ta down to the overageing temperature Toa includes at least one spraying of liquid or a projection of coolant gas onto the strip or the placing of this strip in contact with a chill roll.
The cooling procedure is particularly advantageous when it is desired to achieve a cooling rate that is high enough for the entire process to be able to be carried out in compact lines with high production efficiency.
According to one embodiment of the process which is the subject of the present invention, after annealing, a continuous overageing treatment is carried out by cooling the said strip to an overageing temperature Toa of between 350xc2x0 C. and 450xc2x0 C., maintaining the strip at the overageing temperature Toa for a period of between 40 seconds and 2 minutes, and finally cooling it to a temperature below 100xc2x0 C.
According to another embodiment of the process which is the subject of the present invention, after annealing, the said strip is cooled to an overageing temperature of between 150xc2x0 C. and 250xc2x0 C., the said strip is wound up to form reels, which are introduced, at a temperature of between 130xc2x0 C. and 230xc2x0 C., into a tunnel furnace under a protective atmosphere to prevent oxidation of the said reels, and the said reels are kept in the said tunnel furnace until they have cooled to a temperature below
The above embodiment makes it possible to replace the conventional overageing treatment carried out continuously, that is to say with isothermal maintenance of the travelling strip for 40 seconds to 2 minutes at a temperature of between 350xc2x0 C. and 450xc2x0 C., with a non-isothermal alternative treatment of the strip in the form of a reel placed in a tunnel furnace with a non-oxidative protective atmosphere in which it undergoes slow cooling to a temperature below 100xc2x0 C. during which the soluble carbon precipitates as carbide.
According to another embodiment, after the cooling which follows the respective operation either of overageing with maintenance at the temperature Toa or of passage in the form of reels through a tunnel furnace, the strip is subjected to a xe2x80x9cskin passxe2x80x9d rolling operation with a reduction ratio of between 0.5% and 2.5%.